Abstract
Nitrogen is an integral element of foliar sprays during apple fruit formation. However, in red cultivars, the application may have adverse effects on color in the second stage of fruit development. The effect of a low-dose foliar application of urea was monitored on ‘Topaz’ apple pomometric (fruit mass, firmness, total solids, starch content, skin color) and biochemical traits (primary metabolites, phenolic compounds, and assimilatory pigments). Three applications (17 July, 27 July, and 18 August) of two different urea doses (N1 = 1.4 kg N ha−1; N4 = 4 kg N ha−1) and control (N0, only sprayed with water) were used. Low doses of urea did not affect flesh firmness, total soluble, solid, and starch content, but individual fruit mass was significantly higher in N1 treatment compared to the other two treatments. Significantly lower a* parameters and a lower content of anthocyanins were measured on apples subject to N4 treatment. Many secondary metabolites (phenolic acids, flavan-3-ols, flavonols, and chalcones) were also negatively affected by low-dose urea application. Conversely, urea treatments significantly increased total assimilatory pigments in apple skin. Even minor levels of nitrogen, applied directly on the leaf at later stages of red apple fruit development, negatively altered color traits and the composition of metabolites.
Highlights
Foliar application of urea is a common practice in apple orchards, in conditions unfavorable for soil fertilization
Low levels of nitrogen are often incorporated in preharvest foliar sprays which are used to improve fruit quality, delay fruit ripening, or decrease postharvest fruit decay [11]
The effect was significant on the content of secondary metabolites, which are linked to many defense mechanisms in plants and importantly contribute to visual attributes or health-promoting properties of apple fruit
Summary
Nitrogen is one of the key macronutrients regulating the growth and development of all plants, including apple fruit trees in production orchards. Its availability is linked to vegetative growth as well as to a general increase in biomass, leading to improved plant productivity and fruit yield [1]. Xia et al [2] demonstrated that increased N supply improved leaf N status and photosynthetic activity generating faster cell proliferation and larger ‘Gala’ apples. Nitrogen nutrients added at specific stages of a plant’s development may inhibit the biosynthesis of selected groups of secondary metabolites [3]. These compounds are associated with several major quality traits of apple cultivars and play an important role in plant defense mechanisms [4]
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